The present invention relates to a heterodyne laser spectroscopy system. Laser heterodyne control systems are known in the prior art which use phase locked loop techniques to directly phase lock two or more lasers. However, in the prior art, the original phase lock concepts are used to lock lasers to identical frequencies. This approach, however, does not permit the controlled laser's frequency to be set at any value near that of a reference laser and still remain stable or as accurate as the frequency of the reference laser. This limitation is hence not acceptable for purposes of laser isotope separation spectroscopy, atomic vapor diagnostics, processing of precise laser frequency offsets from a reference, and the like.